5a Arc Flash Study Opening Section.pdf

Arc Flash Study

Arc Flash Study Page 1

Introduction

Arc Flash Studies are conducted as a predictive means for estimating the exposure personnel may have to energy released from an electrical fault. From these calculations a determination can be made of the protective measures personnel should take to work on energized equipment, or whether protective measures are feasible. Generally protective measures consist of Personnel Protective Equipment (PPE) which can be from gloves, face shields, and flame resistant clothing to arc flash suits.

When arc flash hazards are analyzed, the software program determines the theoretical arcing current for each faulted bus and the associated over-current protective device clearing time. This information is then used to calculate the hazard for each faulted bus by determining the amount of energy that can be released. The results can range from arc flash, spraying of molten metal, and pressure wave all with the potential to permanently injure or kill personnel.

Three-phase fault currents are typically used for the arc flash analysis. The IEEE-1584 equations provide conservative arc flash results for most systems. However, Single Line-to-Ground faults, Double Line-to-Ground and Line-to-Line faults can also be modeled using three-phase method.

Default calculations are ½ cycle momentary results. This is based on the assumption that the arc flash is initiated by user actions (screw driver, dropping tools, etc.) and not random insulation breakdown leading to a longer-term arcing fault. This provides the most reasonable and conservative results for the initial arc energy release thus providing greater protection for personnel. Interrupting, 30 cycle currents and integrated method can also be modeled. The integrated method takes into account the decreasing currents from motor and generators and also from multiple sources being switched out through protective device tripping.

More information on Arc Flash is included in this study and is titled “Arc Flash Concepts”. This section goes more into the overall theory and methods used to calculate arc flash.

Arc Flash Hazard Options

Two standards exist for calculating Arc Flash Hazard, IEEE 1584 – IEEE Guide for Performing Arc- Flash Hazard Calculations, and NFPA – 70E Standard for Electrical Safety in the Workplace.

Worst-Case Arc Flash Hazards is obtained from the arcing time from the upstream protective device of the faulted bus. This is the most accurate method.

IEEE 1584 recommends using two scenarios – one with 100% of estimated arc current and the other with 85 % of estimated arc current. This is due to the fact that arc currents may be random and usually vary by some proportion other than the estimated value. Protective devices’ inverse-time over-current characteristics create longer arcing times for smaller currents than for larger currents. Since the incident energy of arc faults is more sensitive to arcing time than it is to arc currents, it is necessary to give more emphasis to arcing time. IEEE 1584 therefore proposes taking 85% of the initial estimate of the arc current.

The computer software calculates both scenario and automatically reports the worst case of incident energy, thus providing conservative results. When 100% values yield greater arc flash

Arc Flash Study Page 2 incident energy, then the tabulated text results are displayed in black in the Arc Flash Report spreadsheet. When the 85% values yield greater incident energy, the text is displayed in pink.

Arc Flash 2 Second Rule

The study’s Arc Flash calculations are based on IEEE 1584 equations, which can produce unusually large incident energy values. Many variables are factored into these calculations with the two most significant being the magnitude of arcing short circuit current and the duration of the arc flash. The duration of the arc flash is directly related to the clearing time of the protective overcurrent device. Low magnitude fault currents often create conditions where the protective overcurrent device is slower in clearing a fault compared to a high magnitude fault. The resultant energy release from such a fault is influenced more by the slower clearing time of the protective overcurrent device than the available fault current.

IEEE 1584 Section B.1.2 states: “If the time is longer than two seconds, consider how long a person is likely to remain in the location of the arc flash. It is likely that a person exposed to an arc flash will move away quickly if it is physically possible and two seconds is a reasonable maximum time for calculations. A person in a bucket truck or a person who has crawled into equipment will need more time to move away.”

This study has made use of this reasoning but has extended the maximum time for calculations to 4 seconds. By doing so many locations in the system have a much lower cal/cm2 value and thus a corresponding lower PPE requirement. This methodology needs to be evaluated against the facility’s own safety policies to determine if there are any conflicts

Equipment Main OCPD

The computer software offers two options for reporting arc fault energy at the incoming of equipment with main over-current protective devices (OCPD). One is to report the incident energy from the secondary side of the main OCPD. Generally the main OCPD is a smaller device and will open on a fault quicker than an upstream device. This can result in a lower incident energy exposure to personnel and a lesser degree of PPE required. This approach ignores the presence of greater exposure from the termination of incoming cables that are not protected by the main OCPD nor isolated by protective barrier. To classify the equipment taking into account the main OCPD may mislead personnel of the greater exposure produced by the incoming cable terminations. Therefore, this study opted to label equipment based on the greater risk created by the incoming main cable terminations.

Report Observations and Recommendations.

1. As mentioned above, labeling equipment based on the exposure from the incoming cables and dependency of the upstream protective device results in a good number of panels and equipment as being “extremely dangerous”. In most cases this equipment is directly fed from the outside pad-mounted transformers that do not provide any secondary protection. The transformer protection is solely from the primary fuses which do not provide tight secondary fault protection. It should be noted that not all incidences where equipment is classified as “extremely dangerous” is due to upstream transformer

Arc Flash Study Page 3 situations. Each situation needs to be reviewed independently for the circumstances creating these conditions.

Refer to the Arc Flash Hazard Report included within each Section 1 thru 6 for individual system findings.

2. It is important to note the Arc Flash Study calculations and recommendations are based on manufacturer’s protective device published performance data. Time and environmental conditions have an impact on equipment of this type and periodic routine maintenance is required to confirm the protective devices are preforming in accordance within the manufacturer’s specifications. Typical deviations in protective device performance tend to increase the clearing time of the device for a fault. This in turn increases the available released energy from an arc flash.

Periodic routine maintenance where testing of protective devices is performed insures protective devices are preforming within manufacturer’s specifications. Only when the protective devices are in compliance with the manufacturers published data can the Arc Flash Study’s recommended Personal Protective Equipment (PPE) be considered representative of actual field conditions.

Circuit breakers are often neglected. Most manufacturers, as well as NFPA 70B, recommend that a molded case circuit breaker be operated, opened or closed, either manually or by automatic means at least annually. Exercising breakers helps to keep the contacts clean due to wiping action and ensures the operation mechanism moves freely.

It is not uncommon for the grease used in breakers to harden, rendering the breaker inoperative. Manual exercising does not operate the mechanical linkage of the trip mechanism. This must be done by current injection tests which at the same time can validate the breakers’ operating time and compliance with factory standards.

A stiff or sticky mechanism, generally caused by the hardening of grease, can cause an unintentional time delay in a breaker’s operation under fault conditions. This could dramatically increase the arc/flash incident energy level to a value in excess of the rating of personal protective equipment.

To illustrate this point, several studies, including IEEE, have shown that low-voltage power circuit breakers, which were not maintained within a 5-year period, have a 50% failure rate.

3. While there are methods to mitigate the impact contributed to arc flash, there is no single answer for every situation. Each situation requires separate analysis and consideration to the cost to implement versus any derived benefit.